Nine miles northwest of Flagstaff on a dirt forest road, a hole in the ground drops into one of the more remarkable geological features in Arizona — a mile-long lava tube formed 700,000 years ago when a volcanic eruption from a nearby Hart Prairie vent sent a river of molten basalt flowing across the landscape. The outer surface of the flow cooled and solidified first. The molten rock inside continued moving, eventually draining out and leaving the hollow tube behind. What remains is a dark, cathedral-quiet underground corridor through the volcanic geology of the San Francisco Peaks volcanic field — the same geological forces that built Humphreys Peak and shaped the high country north of Flagstaff.

Eight of us visited on April 2, 2018 — Luca and I, my sister Natasha and her family. A group that size in a lava tube makes for a genuinely different experience from a solo visit. The cave is big enough to absorb a group of eight without feeling crowded — which tells you something about its scale.

The Descent

The entrance is a scramble down — the opening requires a careful step in and descent rather than a casual walk. The entrance area is wet and the rocks are slick — appropriate footwear and attention to footing on the way in is not optional. We wore jackets and were glad we did. The cave maintains a constant temperature of approximately 42°F regardless of what it is outside — in April that means the cave is roughly the same temperature as the air above, but underground cool feels different from above-ground cool and the jackets earned their place. We did not find ice on the April visit — ice can form near the entrance in winter and persist into spring in cold years, but April 2018 did not produce any.

The Cave

Once inside and past the entrance scramble the cave opens into a genuine underground corridor — high-ceilinged in sections, lower in others, the basalt walls and ceiling showing the textures of the volcanic event that formed them. Small wave-like undulations in the floor are the remains of ripples frozen in the last trickle of molten rock that moved through. Stone formations hanging from the ceiling show where a final heat event caused the rock to partially re-liquefy and drip before cooling permanently in place. The geology is written directly in the surfaces of the cave — you are reading the record of a volcanic event that happened before humans were in North America.

The scale is the thing that stops you. If you have visited Tabernacle Hill Lava Tubes in Utah — as we have, on our 2017 trip — the difference in scale is immediately apparent. The Utah tubes are impressive. Lava River Cave is substantially larger in both length and passage size — a mile of accessible tube compared to the shorter Utah system, with ceiling heights and corridor widths that make the experience feel genuinely subterranean rather than just a tight crawl-through.

The cave continues through to the other end — it is a through-tube rather than a dead end — though we went in and came back out the way we entered. The full through-trip requires commitment to the full mile and planning for exit at the far end. Either approach is valid.

Luca — Barefoot

Shortly after we started into the cave Luca's shoes deteriorated — the sole separating from the upper in the way that cave moisture and volcanic rock accelerates. He hiked much of the cave barefoot. Barefoot on basalt in a 42°F underground corridor — a fact that says more about Luca's determination to complete the cave than it does about the condition of his footwear. He made it through. The shoes did not.

The lesson: check your footwear before the descent. The volcanic rock in lava tubes is hard and sharp in sections. Appropriate shoes for the people who will be wearing them is worth verifying at the trailhead.

What to Bring

This is the most important practical section for anyone planning a Lava River Cave visit:

Two to three light sources per person — not one, not shared. One mile from the entrance in total darkness is a different situation from a short cave where you can see the exit. Each person carries their own headlamp plus a backup. Batteries checked before the descent.

Warm layers — the 42°F constant temperature is manageable with a jacket and is unpleasant without one. In summer the temperature contrast between the 90°F+ above-ground Flagstaff area and the 42°F cave makes it feel genuinely cold. Bring a layer regardless of the outside temperature.

Sturdy shoes with intact soles — see Luca's experience above. The basalt floor is uneven and sharp in sections. Footwear that will survive a mile of volcanic rock in wet conditions.

The Road In

FR 245 and FR 171 are dirt forest roads that may be closed due to snow or mud conditions outside of the late spring through fall window. The roads are passable for most vehicles in dry summer conditions. In winter and early spring the 9-mile drive from US 180 may not be driveable — in which case the cave is accessible via a 4-mile one-way ski or hike from US 180 for those prepared for it. Check road conditions with the Flagstaff Ranger District before the drive.

More Information:

• Bring minimum 2 to 3 light sources per person — headlamps strongly recommended

• Wear warm layers — constant 42°F underground regardless of outside temperature

• Cave is approximately 1 mile long — through-tube with exit at far end or return the way you entered

• Do not touch or deface cave surfaces

• FR 245 and FR 171 typically closed November through April due to snow and mud

#LavaRiverCave #Flagstaff #CoconinoNationalForest #LavaTube #ArizonaCaves #CaveExploring #AZPlaces #CoconinoCounty #ArizonaGeology #SanFranciscoPeaks #FR245 #ArizonaHiking #UndergroundArizona #VolcanicGeology #FreeCamping